This invention relates to a seat belt retractor assembly.
A seat belt for a passenger vehicle often includes a seat belt retractor that retracts the seat belt to remove slack when drawn over a passenger or, alternatively, to withdraw the seat belt when no longer in use. A coil spring generally provides the necessary rewinding or retraction force for the retractor. When the seat belt is drawn over a passenger, the rewind force from the retractor typically increases. In some systems, the amount of force may be objectionable to the vehicle occupant.
Applicant has developed a tension reducing feature that reduces this rewinding force from the seat belt retractor. The details of this seat belt retractor are disclosed and hereby incorporated into this disclosure by U.S. Pat. No. 5,934,595 to Kohlndoffer, et al. Essentially, this tension reducing feature allows for a variable output of the retraction force from the seat belt retractor. One embodiment of the invention uses a cable to couple the seat belt retractor to a reel on which the seat belt is rewound. The cable itself is rewound upon a cone having varying diameters. The cone is linked to the retractor. As the seat belt is drawn out, the cable draws out the spring of the seat belt retractor. Because the cable is wound around the cone of the retractor, the retraction force may also vary.
Current seat belt systems may also include a so-called “pretensioner.” The pretensioner serves to remove slack in a seat belt in the event of a vehicle accident. The pretensioner removes the slack more quickly than a seat belt retractor. A pretensioner may be located either at the belt buckle or the seat belt retractor. For example, upon the detection of a crash, a buckle pretensioner has a gas cylinder that pushes a piston. The piston is linked to the seat belt and quickly applies a tensioning force to the seat belt in the direction of the belt buckle so as to remove slack in the seat belt across the passenger.
When a pretensioner is used with a tension reducing feature, a design challenge arises. When used with a buckle pretensioner, the rapid acceleration of the retractor can cause high stress and breakage in the cable of the tension reducing feature. Moreover, when the retractor locks up during the buckle pretensioner event, the rotational momentum of the tension reducing cone may cause it to over travel and thereby induce slack in the cable, which may foul the retractor. For a retractor pretensioner, the rapid acceleration of the spool may cause the tension reducing cone to create slack in the cable and also foul the seat belt retractor.
A need therefore exists for a seat belt retractor assembly that permits tension reduction but avoids the problems that may arise when the pretensioner is activated.